This invention is concerned with a process for selectively separating copper and zinc values from aqueous solutions containing these metals in salt form. More particularly, the present invention is concerned with a process for selectively recovering copper and zinc from aqueous solutions such as leach liquors and mine waters containing the copper and zinc as sulfate salts. The zinc and copper are separated in the form of aqueous salt solutions, which solutions may be further processed to obtain the metals.
Copper and zinc are found in many ores which can be separated and recovered by solubilizing the copper and zinc in the ore. Generally this is accomplished by roasting or leaching the ore with sulfuric acid to yield a solution containing zinc and copper sulfates. In the past, copper metal was recovered from this solution by electrowinning. The zinc was also recovered by electrowinning provided all the copper was first removed from solution. In order to remove all copper from solution, a series of electrolyses were performed in which a portion of the copper was first removed as pure metal from the solution by electrolysis. The remaining solution from this step was further depleted of copper by a second electrolysis which gave a less pure copper deposit at low current efficiency. Final traces of copper were then removed by cementation with zinc dust.
Another method for concentrating and recovering copper and zinc from aqueous solutions such as leach liquors involves the use of a water-immiscible organic extractant such as an organic hydrogen phosphate. In U.S. Pat. No. 2,992,894, for example, a process for concentrating and recovering copper and zinc from aqueous sulfate solutions is described wherein the solution is contacted with an organic hydrogen phosphate having at least one and not more than two hydrogens of the phosphoric acid replaced with an alkyl radical such as di-(2-ethylhexyl) phosphoric acid to produce a metal cation-loaded solvent which is separated from the solution. The solvent is then stripped with acid or base to produce a metal concentrate and regenerate the solvent. The metal concentrate in the stripping solution is then subjected to electrolysis to obtain the metals. The pH of the aqueous solution in this method must be such that the distribution coefficient for the metal to be extracted is at least 1 and not in excess of that pH value at which a metallic hydroxide precipitates. The preferred pH for copper extraction is 2.5 to 3.5 and the preferred pH for Zn is 1.5 to 4.0 according to this method.
However, the use of organic hydrogen phosphates such as di-(2-ethylhexyl) phosphoric acid to recover zinc from aqueous solutions containing zinc and copper sulfate results in an exchange of hydrogen ions in the organic phase for zinc ions in the aqueous phase. This produces an increase in acidity of the system and results in reduced zinc extraction since the exchange of these ions in the system is pH dependent.
Therefore, since a mole of acid is transferred to the aqueous phase for every mole of zinc or copper extracted into the organic phase, effective metal extraction is only achieved under one of two conditions. These are: (a) metal extractions from dilute solutions which result in relatively small amounts of metals being extracted with accompanying small pH changes; or (b) in systems which incorporate in-situ neutralization of the acid produced during extraction. However, the majority of hydro-metallurgical process schemes involve fairly concentrated metal salt solutions which therefore mandate an acid neutralization operation to satisfactorily utilize extractants such as organic hydrogen phosphates. While acid neutralization is technically feasible it has at least two economic drawbacks; the loss of organic reagent, and the loss of metal values trapped in any insoluble by-product of neutralization. Typically, lime is the acid neutralization reagent of choice in most industrial operations, because of cost and availability. The by-product of acid neutralization in a sulfate system using lime is a relatively insoluble solid (calcium sulfate), which is difficult to handle. More importantly, however, is the fact that insoluble by-products have the disadvantage of trapping expensive organic reagent including the copper and zinc organic phosphates in the calcium sulfate sludge that is produced. In addition, some amounts of copper and zinc hydroxides tend to be produced which are trapped with, and lost to, the calcium sulfate. The calcium sulfate precipitate requires a considerable amount of washing if entrained zinc and copper values are to be removed.
In still another method described in U.S. Pat. No. 3,573,182, selective extraction of zinc from solutions containing both zinc and copper ions is accomplished by using the calcium salt of an organic hydrogen phosphate in an organic solvent as the extractant. In this method a solution of copper and zinc sulfate is mixed with the calcium salt of di-(2-ethylhexyl) phosphoric acid, (Ca(DEP).sub.2), in kerosene. Calcium sulfate precipitates from the mixture with the formation of Zn(DEP).sub. 2 and Cu(DEP).sub. 2. The Cu(DEP).sub. 2 reacts with ZnSO.sub.4 in the aqueous phase to form Zn(DEP).sub. 2 and CuSO.sub.4 with the Zn(DEP).sub. 2 being transferred to the organic phase. The organic phase is then scrubbed with acid to yield ZnSO.sub.4 and di-(2-ethylhexyl) phosphoric acid (DEPH). The aqueous phase containing copper sulfate and the zinc sulfate obtained by scrubbing is subjected to electrolysis to obtain copper and zinc metal respectively.
By using calcium di-(2-ethylhexyl) phosphate according to this method, no hydrogen ions are transferred to the aqueous phase. However, an inherent disadvantage of this process is the formation of a calcium sulfate precipitate which as previously discussed may entrap organic reagents and should be avoided.
The present invention alleviates the problems associated with the aforementioned prior art processes by providing a method for selectively recovering zinc and copper from aqueous solutions containing these metals in salt form, preferably as sulfates, with a water-immiscible organic extractant, which method does not involve a transfer of hydrogen ions from the organic phase to the aqueous phase and does not include the formation of a precipitate as a by-product. Thus, the present method offers the following advantages:
1. Acid neutralization is not required during extraction.
2. Losses of entrained organic extractant and zinc and copper values in a precipitate such as calcium sulfate is eliminated since only free acid in the aqueous feed solution needs to be neutralized prior to extraction.